Light emitting diodes (LEDs) have a variety of favorable properties including long life, high brightness, low voltage, small size, an almost complete absence of heat rays during use and good retention of light emitting efficiency even at low temperature.
In a light emitting device such as a light emitting diode (LED), a composition for sealing a light emitting element is required not only to protect the light emitting element from external influences but also to provide satisfactory and stable adhesiveness of the light emitting element to a support substrate of polyphthalamide, ceramics, or the like supporting the light emitting element. The composition needs also to have high transparency so as not to decrease luminance of the light emitting element.
As such a sealing composition, for example, epoxy resin and the like have been conventionally used. However, recently LEDs have become more and more efficient resulting in increased luminance, increased heat generation during use and emission of light of shorter wavelength, and thus the use of the epoxy resin has been a cause of cracking and yellowing.
Therefore, organopolysiloxane compositions (silicone compositions) have been used because of their excellence in heat resistance and ultraviolet resistance. In particular, addition-reaction type silicone compositions curing via hydrosilylation reaction have been widely used since they are excellent in productivity because of their quick curability by heating and avoidance of formation of any byproduct when cured.
Many references deal with such silicone compositions and their use for LED manufacturing.
WO 2008023537 A1 describes a curable organopolysiloxane composition comprising at least the following components: (A) a linear diorganopolysiloxane with a mass average molecular weight of at least 3,000, (B) a branched organopolysiloxane, (C) an organopolysiloxane having, on average, at least two silicon-bonded aryl groups and, on average, at least two silicon-bonded hydrogen atoms in one molecule, and (D) a hydrosilylation reaction catalyst; this composition has excellent curability and, when cured, forms a flexible cured product of high refractive index, optical transmissivity, excellent adherence to various substrates, high hardness and slight surface tack.
EP 2032653 B1 describes a curable organopolysiloxane composition comprising at least the following components: an organopolysiloxane (A) represented by the following general formula: R13SiO(R12SiO)mSiR13 (where R1 is a monovalent hydrocarbon group, and “m” is an integer from 0 to 100); an organopolysiloxane (B) represented by the following average unit formula: (R2SiO3/2)a(R22SiO2/2)b(R23SiO1/2)c (where R2 is a monovalent hydrocarbon group, and “a”, “b”, and “c” are specific numbers); an organopolysiloxane (C) having in one molecule on average at least two silicon-bonded aryl groups and on average at least two silicon-bonded hydrogen atoms; and a hydrosilylation-reaction catalyst (D); this composition is characterized by good fillability and curability and, upon curing, forms a cured body that possesses a high refractive index, high light transmissivity, and strong adhesion to various substrates.
US 20070249790 A1 describes a colorless transparent silicone lens produced by thermally curing a silicone resin composition comprising (A) an organopolysiloxane having a resin structure comprising R1SiO1.5 units, R22SiO units, and R3aR4bSiO(4-a-b)/2 units, wherein R1, R2, and R3 are independently a methyl group, an ethyl group, a propyl group, a cyclohexyl group, or a phenyl group, R4 is a vinyl group or an allyl group, a is 0, 1, or 2, b is 1 or 2, and a+b is 2 or 3, and wherein the number of repetition of the R22SiO unit is 5 to 300, (B) an organohydrogen polysiloxane having a resin structure comprising R1SiO1.5 units, R22SiO units, and R3cHdSiO(4-c-d)/2 units, wherein c is 0, 1, or 2, d is 1 or 2, and c+d is 2 or 3, and wherein the number of repetition of the R22SiO unit is 5 to 300, and (C) a platinum group catalyst. This composition shows excellent flexibility, transparency, and moldability as well as reduced surface tackiness is provided.
US 20080160322 A1 describes a silicone composition for sealing a light emitting element including (A) a vinyl group-containing organopolysiloxane having a three-dimensional network structure represented by the average unit formula (SiO4/2)a(ViR2SiO1/2)b(R3SiO1/2)c, wherein Vi represents a vinyl group, R's are identical or different substituted or unsubstituted monovalent hydrocarbon groups other than alkenyl groups, and a, b, and c are positive numbers satisfying that a/(a+b+c) is 0.2 to 0.6 and b/(a+b+c) is 0.001 to 0.2; (B) an organohydrogenpolysiloxane containing at least two hydrogen atoms per molecule, each hydrogen atom being bonded to a silicon atom, the organohydrogenpolysiloxane being contained in such an amount that the amount of hydrogen atoms bonded to a silicon atom is 0.3 to 3.0 mol per 1 mol of a vinyl group bonded to a silicon atom in the component (A); and (C) a catalytic amount of a hydrosilylation catalyst, wherein the coefficient of linear expansion of the composition after curing is 10×10−6 to 290×10−6/° C. It is possible to obtain a cured product whose residual stress with a support substrate is reduced and which has long-term satisfactory and stable adhesiveness.
EP 1767580 A1 discloses an addition reaction curing silicone composition, comprising an organopolysiloxane, wherein some of the silicon atoms may be linked via bivalent hydrocarbon groups of 2 to 10 carbon atoms. However, at least 80 mol-% of all groups bridging two silicon atoms have to be oxygen atoms, i.e. the polymeric backbone predominantly consists of siloxane units with only few silalkylene or silarylene units being allowed. The composition is said to be particularly useful as a sealing material for photo devices such as LEDs, as well as lens material or hard coating agent or the like.
KR 20100030959 A teaches polysiloxanes for sealing materials or a lens of a LED. The polysiloxanes comprise a repeating unit [—Si(R1)(R2)-X1-C6H4—Y1-]m, wherein R1 and R2 are independently H, methyl, ethyl, phenyl or C2-C6 alkenyl; X1 and Y1 are independently C0-C6 alkylene, C2-C6 alkenylene, C2-C6 alkynylene, NH or O; and m is 1 or more. The presence of siloxane units is not excluded.
From WO 2009/131023 A1 silicon-containing polymers are known comprising -R2-C6H4-R2- units bridging neighboring silicon atoms, wherein R2 designates identical or different, substituted or unsubstituted alkylene groups. The silicon-containing polymers further comprise siloxane units. The molar amount of units comprising said -R2-C6H4-R2- bridges within the silicon-containing polymer is below 10%. The polymers may be used in curable polymer compositions suitable for obtaining a cured product characterized by a high index of refraction. Such compositions are useful as a sealant for optical devices such as LEDs and for use in optical instruments.
From the above documents, it can be seen that silicone compositions are widely used as LED encapsulant material. However, since silicone compositions have large free volume as an inherent property, they show much worse gas and moisture barrier properties than epoxy based materials. Thus, moisture penetrates more easily into the encapsulants resulting in the corrosion of LED chips, which greatly influences the durability of LEDs.
Therefore, it is a great challenge to develop an LED encapsulant with both very good gas and moisture barrier properties and at the same time thermal stability equal to the stability of silicone compositions.